Method for operating a water electrolysis device

A method for operating a water electrolysis device produces hydrogen and oxygen from water. In the electrolysis device, water which comes from a PEM electrolyzer is fed to a first heat exchanger for cooling, subsequently to an ion exchanger, then to a second heat exchanger for heating and again to the PEM electrolyzer, in a water circuit. The heat exchangers form part of a common heat transfer medium circuit at a secondary side. The heat transfer medium circuit includes a cooling device, through which the heat transfer medium flow is selectively led in a complete or partial manner or not at all, for the control and/or regulation of the temperature of the water which is fed to the ion exchanger and/or to the PEM electrolyzer.